Communications satellites typically include one or more antenna assemblies for communicating with various terrestrial target devices, which may include ground-based access node terminals or user terminals, any of which may be stationary (e.g., installed at a permanent installation site, moved from one fixed installation site to another, etc.) or mobile (e.g., installed at a vehicle, a boat, a plane, etc.). An antenna assembly of a communications satellite may be configured for transmitting downlink signals (e.g., forward link signals to user terminals, return link signals to access nodes) and/or receiving uplink signals (e.g., forward link signals from access nodes, return link signals from user terminals). The antenna assembly may be associated with a service coverage area within which devices may be provided a communications service via the antenna assembly. The satellite may be a geostationary satellite, in which case the satellite's orbit is synchronized with the rotation of the Earth, keeping the service coverage area essentially stationary with respect to the Earth. In other cases, the satellite is in an orbit about the Earth that causes the service coverage area to move over the surface of the Earth as the satellite traverses its orbital path.
Some satellite communication systems employ “bent-pipe” satellites that relay signals among terminals located in the same antenna footprint (e.g., service coverage area), for example, the continental Unites States. In circumstances where transmit and receive coverage areas are overlapping, separate frequency bands and/or polarizations may be used for the uplink (to the satellite) and the downlink (from the satellite). The “bent-pipe” designation refers to the fact that the relayed signals are effectively retransmitted after the signals are received by the satellite, as if redirected through a bent pipe. The data in the relayed signals is not demodulated or remodulated as in a “regenerative” or processing satellite architecture. Rather, signal manipulation on the satellite in a bent-pipe architecture is generally limited to functions such as frequency translation, filtering, amplification, and the like.
Other satellite communication systems were developed around satellites that employ innovations such as digital channelization and routing of signals, demodulation/routing/re-modulation of the data in the relayed signals, narrow antenna footprint spot beams to allow frequency reuse, and phased array antennas to allow dynamic placement of coverage areas.
For example, satellites for Mobile Satellite Services (MSS) typically employ spot beam coverage areas with a greater degree of frequency reuse. Examples of satellites for MSS include the Inmarsat-4 satellites and the Thuraya satellites. These satellites typically feature a large number of narrow spot beams covering a large composite area and allow for flexible and configurable allocation of bandwidth. However, the total system bandwidth is low (such as a 34 MHz allocation at L-band), and service is generally categorized as “narrow band” (e.g., carrier bandwidths of hundreds of kHz), which allows the flexible and configurable bandwidth allocation to be accomplished using digital beamforming techniques. These satellites use a large reflector with an active feed array. The signals associated with each antenna feed element are digitized, and the beamforming and bandwidth flexibility are provided by a digital signal processor. The digital beamforming is performed on narrowband channels, allowing any narrowband channel on the feeder link to be placed at any frequency for any spot beam shape.
The Wideband InterNetworking Engineering Test and Demonstration Satellite (WINDS) is an experimental Ka-band satellite system. The satellite implements both fixed spot beams using a fixed multi-beam antenna (MBA) and steerable beams using an active phased array antenna (APAA). The MBA serves fixed beams, and the communications link can be switched over time in a pattern consisting of combinations of receiving and transmitting beams. The APAA has been developed as a beam-hopping antenna with a potential service area that covers almost the entire visible region of earth from the satellite. The APAA can provision communications between arbitrary users using two independently steerable beams for each of the transmitting and receiving antennas. Beam steering is achieved by updating pointing directions via control of digital phase shifters in switching interval slots as short as 2 ms in Satellite Switched Time Division Multiple Access (SS-TDMA) mode, where the shortest beam dwell time corresponds to the slot time of the SS-TDMA system. Beam switching at high speed is supported for up to eight locations per beam. Switching patterns for both the MBA and APAA are uploaded from a network management center.
Spaceway is a Ka-band satellite system that services 112 uplink beams and nearly 800 downlink beams over the United States. The Spaceway satellite uses a regenerative on-board satellite processor to route data packets from one of 112 uplink beams to one of nearly 800 possible downlink beams. At any time the downlink consists of up to 24 hopping beams. The downlink scheduler determines which beams should be transmitting bursts for each downlink timeslot depending on each beams downlink traffic queue and power and interference constraints.
The Wideband Global SATCOM (WGS) satellite, formerly known as the Wideband Gapfiller Satellite, is a U.S. government satellite that employs steerable Ka-band spot beams and X-band beamforming. The Ka-band spot beams are mechanically steered. Up to eight X-band beams are formed by the transmit and receive X-band arrays using programmable amplitude and phase adjustments applied to beamforming modules (BFMs) in each antenna feed element. Bandwidth assignment is flexible and configurable using a broadband digital channelizer, which is not involved in beamforming.
More recent satellite architectures have resulted in further increases in system capacity. For example, ViaSat-1 and the Ka-band spot beam satellite architectures disclosed in Dankberg et al. U.S. Pat. App. Pub. No. 2009-0298416, which is incorporated by reference herein in its entirety, can provide over 150 Gbps of physical layer capacity. This spot beam architecture provides over an order of magnitude capacity increase over prior Ka-band satellites. Other satellites, for example KA-SAT and Jupiter, use similar architectures to achieve similarly high capacities. The architecture used in all of these satellites is a “bent pipe” hub-spoke architecture that includes small spot beams targeted at fixed locations. Each spot beam may use a large amount of spectrum, typically 250-1000 MHz. The resulting large capacity is a product of several characteristics of the satellite system, including, for example, (a) the large number of spot beams, typically 60 to 80 or more, (b) the high antenna directivity associated with the spot beams (resulting in, for example, advantageous link budgets), and (c) the relatively large amount of bandwidth used within each spot beam.
The aforementioned high capacity satellite architectures are valuable, but may still be limited in certain respects. For example, scaling the architecture to support higher capacities while maintaining the same spectrum allocation and power budget is typically accomplished using larger reflectors to create spot beams with smaller diameters. The use of smaller diameter spot beams may increase the directivity (or gain) of the satellite antenna, thus enhancing the link signal-to-noise ratio (SNR) and capacity. However, the smaller spot beams necessarily reduce the service coverage area (e.g., the coverage area for which a communications service can be provided). These satellite architectures, therefore, have an inherent tradeoff of capacity versus coverage area.
In addition, these architectures typically place all spot beams, both user beams and gateway (GW) beams, in fixed locations. There is generally no ability to move the spot beams around to accommodate changes in the service coverage area. Moreover, the architectures essentially provide uniformly distributed capacity over the service coverage area. The capacity per spot beam, for example, is strongly related to the allocated bandwidth per spot beam, which is predetermined for every spot beam and allows for little to no flexibility or configurability.
Although these satellite communications architectures are valuable when the desired service coverage area is well-known and the demand for capacity is uniformly distributed over the service coverage area, the inflexibility of the aforementioned architectures can be limiting for certain applications. For example, a communications satellite may be retasked or deployment conditions (e.g., orbital slot, etc.) may change. Additionally, a satellite communications service may see changes in user demands (e.g., fixed vs. mobile users, etc.). Although signal processing techniques such as beamforming may provide some ability to adapt the arrangement of spot beams or service coverage area, additional flexibility in adaptation of service coverage area and spot beam arrangement may be desired. For example, it may be desirable for a satellite communications system architecture to support flexibility in the locations and sizes of spot beam coverage areas, the locations of user terminals and access node terminals, the spatial distribution of the communications service capacity, and the capacity allocation of the communications service. Further, it may be desirable to support such flexibility along with changes in orbital position of a communications satellite or allow moving a communications satellite to another orbital slot during the mission lifetime.